The process of heating a pre-molded glass material formed of an optical glass, i.e., a preform and precision press-molding the preform with a press mold to produce an optical element such as an aspherical lens, a microlens, a diffraction grating, or the like is called “precision press-molding process” or “mold optics method”, and is well known.
There are various optical properties that are required of glasses to be used for the precision press-molding, and one of such properties is that an optical glass has an Abbe's number (νd) of 50 to 59. There has been known a phosphate-based optical glass that is not for precision press-molding but has such optical properties, as is a glass described in JP-A-58-25607.
Meanwhile, when attempts are made to materialize a glass that exhibits an Abbe's number (νd) of 50 to 59 and that is suitable for the precision press-molding, on the basis of such a phosphate-based composition disclosed in JP-A-58-25607, there is caused a problem that a reaction between the glass and the surface of a press mold during precision press-molding damages the surface of the precision press-molded article or causes foaming, or that the glass is melt-sticks to the surface of the press mold.
The precision press-molding method has a characteristic feature in that the molding surface of a press mold is precisely transferred to a glass to form an optical-function surface such as a lens surface precisely by press molding. When the above damage or foaming takes place on the surface of a precision press-molded product, it is required to carry out a surface-removing process such as polishing or the like, and the above characteristic feature of the precision press-molding method is impaired. A glass to be used in the precision press-molding method is hence required to have the property of having no or less reactivity with the molding surface.
In the precision press-molding, further, it is required to set the press-molding temperature at a lower temperature than the temperature employed for general glass press-molding, so that a glass to be used for the precision press-molding is required to have the property of being softened at a low temperature (low temperature softening property). Further, the glass is required to have excellent stability for forming a preform from the glass in a molten state without causing the devitrification thereof. Moreover, it is required to impart the glass with higher refractivity for increasing the added value of the glass as an optical glass.
Therefore, the optical glass, which exhibits an Abbe's number (νd) of 50 to 59 and is suitable for the precision press-molding, is required have the property of having no or less reactivity with a molding surface, the low-temperature softening property, excellent glass stability (devitrification resistance) and high refractivity.